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If you'd specified this as a habitable room from the outset, the planners would probably have raised overlooking concerns and asked you to amend the plans accordingly. It does sound like at least some of your architect, builder, and self already had another use in mind, so one has to wonder why this wasn't specified as a habitable room? Look at it from your neighbour's perspective - when you applied for planning did they expect someone looking out over their property from a meter away for 10 hours a day? We all grumble about planners and hope to be treated fairly and that's what this sounds like - the planners let you build a metre from your neighbour and now they may allow your neighbour to do the same. Imagine if your neighbour had built the same extension first? I don't think you'd want to have your own plans refused.

Could you mirror your proposed new layout so you enter through what is currently the cloakroom door? Then the toilet and its plumbing just turns through 90 degrees in the same corner as now.

Agree with @Mr Punter . Just engage positively and promptly unless something seems entirely unreasonable. All you'll achieve arguing over BC approval etc is paying more fees, as the surveyor will bill you by the hour to explain why the information is required. It may not feel like it, but you're still ahead by having agreed to use the same surveyor - even if you'd appointed your own, it sounds like the neighbour would still have appointed this surveyor; then you'd have been paying for two, and potentially a third if they cannot agree. Hopefully the surveyor has already given you some idea of the approximate likely cost of an award, along with their hourly rate.

I like a lot about this layout. I'd be tempted to push the main hall wider and also allow yourself a generous 1000mm staircase,losing the bullnose stair that interrupts the hall flow a bit. I'd consider making more room on the landing by pushing the wall with the master's ensuite back ~750. That does away with the awkward doorway corridor into the master bedroom and gives more space for say, a console table and better people flow on the landing. These are spaces you pass through a lot. You could think about entering the master bedroom via the dressing room - landing door into the dressing room and then double doors from dressing room so you enter the bedroom looking out towards the bifolds / french doors with a juliet balcony. Bifolds are shown as 4+0, you might find 3+1 with a traffic door more practical. I like the lounge/kitchen swap idea to sit looking out to the rear, it'll probably enhance the view through from front to back as you enter the house - lower furniture, no sink & taps.

Are you sure this is the only non-compliance? I expect all the other doors needed upgrading to fire doors -if that's been done then it's a bit surprising that none of your designer, BC or builder (or you) picked up on the lack of any door or fire protection to the open plan area. Agree with others that regardless of blame, you need to focus on the most pragmatic solution - very likely a door downstairs. I don't believe a mist system alone would be complaint, there's a pretty good guide available from Building Control Alliance (see option 2b) that suggests you'd also need a door on the first floor protecting the stairway. You could then ask your designer to cover, say, half the cost of reasonable remedial work like adding a fire door (they're not the only ones to blame here). Small claims is your only cost effective avenue if not; it's not worth legal fees and any liability will be small, not a payout for a fire suppression system.

Measured/design AP50 = 0.3000 (17) in the report, but a simple explanation would be it's a mistype and should be 3 (the pessimistic end of their original range). Definitely something to query with the builder.

As @JohnMo can you hold them to achieving this figure? What's changed in the design to improve airtightness so that it exceeds passivhaus standard? If this figure is over optimistic just to pass the SAP as-designed stage then once built, if you only achieve 1.5 your actual ventilation heat losses will be 5 times as much, hurting your SAP rating and your pocket! Useful guide to good airtightness practice here Heating efficiency of 219% will be taken from the Product Characteristic Database - just look up your heat pump model (anecdotally, the fairly standard Mitsubishi Ecodan we intend to fit has a figure of 267% so there are more efficient units out there) The full SAP procedure guide is a bit overwhelming but covers the calculations in excruciating detail. I'm no expert but managed to dig out specific details that impacted our report. All the similar-but-different sections take much of the same base information and then apply it separately to figure out different parts of the assessment - carbon emissions, energy costs etc. For example, a gas boiler is worse than ASHP for emissions but still good for running costs. The section you're probably most interested in is 'Calculation of Energy Rating' (starts page 15 of yours) as that determines the A/B/C rating that is the only thing most buyers will care about. Your space heating cost will rise a lot if you miss that airtightness figure. Are you installing any renewables? Solar or WWHRS? If so I'd expect those to show up as a negative figure, reducing energy costs, under 'Energy saving/generation technologies' on page 18

Love a free standing bath but unless you widen the ensuite it will look squeezed in, not luxurious. A bit bigger bedroom will give more freedom for furniture layout too. See rough layout below. Here's an example (not mine sadly!) of the sort of ensuite I think you could achieve with a bit more space.